Image: Artist’s rendering of Haumea (SINC/José Antonio Peñas).
Haumea (formerly (136108) 2003 EL61) is probably one of the most interesting and intriguing Trans-Neptunian Objects. Haumea presents various atypical characteristics: it is large, bright, fast rotator, it has pure water ice on the surface, has a dark red spot on the surface, has a high density, has at least two satellites and there are more than ten Trans-Neptunian Objects with very similar orbital parameters and similar surface properties to Haumea. Haumea, its two moons and all bodies dynamically related with this object form a peculiar family in the Trans-Neptunian belt. From the short-term variability studies, one can appreciate that Haumea is the fastest rotator known to date, close to a spin barrier suggesting that this object may have suffered a rotational fission.
The Haumea family is composed of objects sharing similar proper orbital elements and similar surface properties. Currently, the list of confirmed Haumea family members is: (24835) 1995 SM55, (19308) 1996 TO66, (86047) 1999 OY3, (55636) 2002 TX300, (120178) 2003 OP32, 2003 SQ317, 2003 UZ117, (308193) 2005 CB79, (145453) 2005 RR43, (136108), 2009 YE7, Haumea, Namaka, and Hi’iaka. Some objects have proper orbital elements consistent with the family but without any near infrared spectra confirming the presence of water ice on their surfaces. These objects are known as candidates. Some of these candidates have been rejected based on colour studies. However, one object, named 2008 AP129 has proper elements consistent with being a member of the family but does not have a strong water ice signature. It is speculated that this object could be a fragment from an inner part of a differentiated proto-Haumea or a fragment from the partially differentiated icy/rocky mantle of the pro-Haumea. The term ”proto-Haumea” is used to refer to the object prior to the formation of the family. The name ”Haumea” is used to refer to the actual object.
Recently, we have investigated the rotational properties of the family members and unconfirmed family candidates with short-term variability studies, and report the most complete review to date. The mean rotational periods, from Maxwellian fits to the frequency distributions, are 6.27±1.19 h for the confirmed family members, 6.44±1.16 h for the candidates, and 7.65±0.54 h for other TNOs (without relation to the family). According to our study, there is a suggestion that Haumea family members rotate faster than other TNOs, however, the sample of family member is still too limited for a secure conclusion.
Figure 1: It is thought that the proto-Haumea (large object in the center of the figure) was a differentiated or partly-differentiated object with a rocky interior and an icy/rocky mantle. Most of the icy/rocky mantle has been removed and the fragments ejected created the Haumea family members (grey objects). The leftover of the proto-Haumea is the current Haumea. Several objects are classified as candidates to the family (red objects). They are not official members of the family because their composition is unknown or because they have no ice on their surface as the other members of the family. Shape of these objects is approximate and has been derived from their lightcurves. In some cases, objects have never been observed for lightcurves and so we do not have a good constrain about their shape. These objects are indicated with a question mark.
I also participated on the elaboration of a model able to explain the Haumea family genesis thanks to rotational fission triggered by a sub-catastrophic collision.
First case: Rotational fission by increasing the angular momentum
The first step of our model is to test the feasibility of the rotational fission. In other words, it is necessary to test the object’s disruption limit. For this purpose, we increased the angular momentum of the synthetic object by twenty-one small increases of the angular momentum until the fission occurred. Each spin up corresponds to an increase of 1% of its angular momentum. After each increase, we allowed the object enough time to adjust itself to the corresponding rotational figure of equilibrium. The simulation corresponding is:
Figure 2: Different colors are used every time the object is spun-up. The initial target (in red) suffered several increases of angular momentum. The target
deformation is noted until its break up (in blue).
The main result of this simulation is the formation of a binary system and the confirmation that the rotational fission is feasible for this kind of object.
Second case: Rotational fission triggered by a low speed sub-catastrophic collision
Now, we want to check if a low speed collision is able to provoke the rotational fission. We create a spherical projectile with a density around 1 g/cc. We perform a collision with a velocity of 1 km s−¹. The simulation is:
Figure 3: Rotational fission triggered by a sub-catastrophic collision at low impact velocity.
After the collision, one part of the projectile is encrusted on the target surface whereas the rest is ejected at high velocity. This feature may explain the dark spot reported on the current Haumea surface. We must point out that part of the target is also ejected during the collision. The quantity of target material ejected depends, basically, on the impact velocity. Due to its own rotation, the target becomes more and more deformed until it reaches a “skittle-form”, known as Poincaré figure. Finally, due to its own rotation, the “head and the body of the skittle” are separated. The “head” becomes a satellite of the largest remnant.
Third case: Rotational fission triggered by a high-speed collision
A similar result is obtained with a higher impact velocity, 3 km s−¹. Corresponding simulation:
Figure 4: Rotational fission triggered by a sub-catastrophic collision at higher impact velocity.
For more details, please see [.pdf]
Audrey Thirouin 